The embodiments of the invention provide a metal capping process for a back end of line (BEOL) interconnections.
As wire widths in integrated circuits continue to shrink, the electrical conductivity of the wiring material becomes increasingly more important. The material of choice since the integrated circuit art began, aluminum, is becoming less attractive than other materials, such as gold, copper, and silver, which are better conductors. In addition to possessing superior electrical conductivity, these materials are more resistant than aluminum to electromigration, a property that increases in importance as wire cross-sectional areas decrease and applied current densities increase. In particular, copper is seen as an attractive replacement for aluminum because copper offers low cost, ease of processing, lower susceptibility to electromigration, and lower resistivity.
Metal caps on copper lines improve the electromigration lifetime of the copper significantly. However, a selective metal cap deposition is often difficult to manufacture. Thus, a feasible way to deposit metal caps on copper lines is needed. Further, with metal caps, the current density in the line can be much higher than is desired. Also, some conventional techniques, such as an electroless selective metal capping process, leave a residue and usually produce shorting and reliability problems, especially at thin wire levels.
Thus, this disclosure presents a way to create metal caps on copper lines within an inter-line dielectric (ILD) by depositing a thin (e.g., 5 nm) metal blanket film (e.g., Ta/TaN) on top of the copper lines and dielectric, after the wafer has been planarized in for example, a chemical mechanical planarization process (CMP). Further a thin dielectric cap is formed over the metal blanket film. A photoresist coating is deposited over the thin dielectric cap and a lithographic exposure process is performed, but without a lithographic mask. A mask is not needed in this situation, because due to the reflectivity difference between copper and the ILD lying under the two thin layers, a mask pattern is automatically formed for etching away the Ta/TaN metal cap between copper lines. Thus, this mask pattern is self-aligned above the copper lines.
Since the metal cap deposition is a blanket metal cap deposition, all copper surface will be covered independently of underlying copper line pattern density. One advantage of such processing is that no lithographic mask is required to pattern the photoresist. In addition, with embodiments here no residual is left, unlike conventional electroless processes, which leave a residue. Further, materials such as Ta/TaN for the metal blanket film can stand the stripping process of the remaining photo resist after the etching process is complete.
These and other aspects of the embodiments of the invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating embodiments of the invention and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments of the invention without departing from the spirit thereof, and the embodiments of the invention include all such modifications.